1. Field of the Invention
This invention relates generally to electronic camera modules, and more particularly to miniature digital camera modules.
2. Description of the Background Art
Digital camera modules are currently being incorporated into a variety of host devices including, but not limited to, cellular telephones, personal data assistants (PDAs), computers, etc. Therefore, consumer demand for digital camera modules in host devices continues to grow.
Host device manufacturers prefer digital camera modules to be small, so that they can be incorporated into the host device without increasing the overall size of the host device. Not only do host device manufacturers prefer camera modules that are small, but also camera modules that capture images of the highest possible quality. Therefore, it is an ongoing goal for camera module manufacturers to design camera modules that meet these requirements and can be produced at minimal manufacturing cost.
A conventional digital camera module generally includes an integrated image capture device (ICD), a printed circuit board (PCB), a housing, and a lens unit. Typically, the components are formed separately and later assembled to create the digital camera module. That is, the ICD is flip-chip mounted to the bottom of the PCB such that the image capture surface of the ICD is aligned with an optical opening formed through the PCB. Then, the bottom of the housing is attached to the top of the PCB over the optical opening so as to enclose the image capture surface. Once the housing is attached to the PCB, the lens assembly is mounted to the opposite end of the housing so as to focus incident light onto the image capture surface. Typically, the lens unit includes some type of sloped surface (e.g., threads, cam, ramps, etc.) that engage a complementary sloped surface formed on the housing such that proper focusing can be achieved by rotating the lens unit within the housing. After the lens assembly is properly displaced with respect to the image capture surface, the lens assembly is fixed (e.g., via adhesive, thermal weld, etc.) with respect to the housing. The ICD typically includes a set of electrical contacts that are electrically connected to an associate set of contacts of the PCB. The PCB typically includes a second set of electrical contacts that connect to the circuitry of the host device so that the ICD can communicate image data to the host device for processing, display, and storage.
Although the particular types of circuit substrates can vary in different camera module designs, ceramic circuit substrates are often preferred. This is because ceramics provides excellent electrical and thermal insulation while also maintaining structural integrity at high temperatures. Indeed, this is desirable to manufacturers because it provides more freedom to implement electrical bonding techniques (e.g., reflow soldering) that require high temperatures.
Although there are benefits to using ceramic circuit boards in camera modules, there are also several disadvantages. As one example, ceramic circuit substrates are relatively difficult and expensive to manufacture. Typically, ceramic circuit boards are manufactured in large volumes wherein multiple circuit boards are formed simultaneously in a coupon format. This involves first forming an array of discrete circuits on a large ceramic board and then singulating the board into a plurality of individual camera module PCBs. The singulation process typically involves scoring the large ceramic board and then snapping it along the score marks. Because of the inherent brittleness of ceramic material, uncontrolled fracture often results in high yield losses.
As another disadvantage, it is difficult to manufacture ceramic circuit boards with a high degree of geometric accuracy because of the inherent warpage associated with ceramics. That is, raw or “green” ceramic material tends to shrink rather unpredictably during the curing or “firing” process. This is particularly problematic in the large scale production of ceramic circuit boards because a relatively high percentage of ceramic substrates inevitably experience warpage that is too severe for use and, therefore, have to be discarded. As a consequence of the inherent warpage and aforementioned fracturing problems associated with ceramics, manufacturers must implement rigorous quality control processes in order to identify the unusable units. Of course, the strict quality control practices and high yield losses associated with ceramic circuit substrates contribute to both a high overall manufacturing cost and slow manufacturing throughput for the camera modules in which they are employed.
In manufacturing ceramic PCBs, completely eliminating warpage is highly unlikely and the units that do satisfy quality control standards typically exhibit some degree of warpage. Although such warpage may be slight, it is typically sufficient to impose several manufacturing challenges and design constraints. For example, establishing a reliable electrical connection during the flip-chip bonding of an ICD onto a ceramic PCB requires forming a double stud bump on each contact of the ICD. Furthermore, the number of processes and materials that are required for double stud bump bonding can be relatively expensive and time consuming. Therefore, it would be desirable to reduce the number of stud bumps required to achieve a reliable electrical connection between the ICD and the circuit substrate.
As yet another disadvantage, ICDs that are bonded to ceramic circuit substrates typically have to be thicker than those bonded to other types of circuit substrates. Of course, this is undesirable to manufacturers because as the required ICD thickness increases, so does the required overall height of the camera module. One reason that ICDs have to be thicker when mounted on ceramic circuit boards is to withstand the higher bonding forces (i.e., thermocompression force). Another reason such ICDs have to be thick is to withstand high stresses that occur during temperature changes. Such stresses result from a relatively large difference in coefficient of thermal expansion (CTE) between the ceramic of the PCB and silicon of the ICD.
What is needed, therefore, is a camera module design that reduces the number of processes and components required for mounting the ICD on the circuit substrate. What is also needed is a camera module that can be manufactured using a thinner ICD. What is also needed is a camera module design that reduces circuit substrate warpage.